NOTE6

It is now legal to have color events, such as (red), in the
portcullis of a book. More generally, it is legal to set the
acl2-defaults-table in the portcullis of a book. For example, if
you execute :red and then certify a book, the event (red) will show
up in the portcullis of that book, and hence the definitions in that
book will all be red (except when overridden by appropriate
declarations or events). When that book is included, then as
always, its portcullis must first be ``raised,'' and that will cause
the default color to become red before the events in the book are
executed. As always, the value of acl2-defaults-table immediately
after execution of an include-book, certify-book, or encapsulate
form will be the same as it was immediately before execution (and
hence, so will the default color). See portcullis and, for
more about books, see books.

The function nth is now enabled, correcting an oversight from
Version 1.5.

Customization files no longer need to meet the syntactic
restrictions put on books; rather, they can contain arbitrary Acl2
forms. See acl2-customization.

Structured directory names and structured file names are supported;
see especially the documentation for pathname, book-name,
and cbd.

Acl2 now works with some Common Lisp implementations other than
akcl, including Lucid, Allegro, and MCL.

A facility has been added for displaying proof trees, especially
using emacs; see proof-tree.

There is a considerable amount of new documentation, in particular
for the printing functions fmt, fmt1, and fms, and for the notion of
Acl2 term (see term).

It is possible to introduce new well-founded relations, to specify
which relation should be used by defun, and to set a default
relation. See well-founded-relation.

It is possible to make functions suggest new inductions.
See induction.

It is possible to change how Acl2 expresses type-set information; in
particular, this affects what clauses are proved when forced
assumptions are generated. See type-set-inverter.

A new restriction has been added to defpkg, having to do with
undoing. If you undo a defpkg and define the same package name
again, the imports list must be identical to the previous imports or
else an explanatory error will occur.
See package-reincarnation-import-restrictions.

Define-pc-macro and define-pc-atomic-macro now automatically define
:red functions. (It used to be necessary, in general, to change
color to :red before invoking these.)

For a proof of the well-foundedness of e0-ord-< on the e0-ordinalps,
see proof-of-well-foundedness. [Note added later: Starting with
Version_2.8, o< and o-p replace e0-ord-< and e0-ordinalp,
respectively.]

When the system is loaded or saved, state is now bound to
*the-live-state*.

Certify-book has been modified so that when it compiles a file, it
loads that object file.

Defstub has been modified so that it works when the color is hot
(:red or :pink).

Several basic, but not particularly commonly used, events have been
added or changed. The obscure axiom symbol-name-intern has been
modified. The definition of firstn has been changed. Butlast is
now defined. The definition of integer-length has been modified.
The left-hand side of the rewrite rule rational-implies2 has been
changed from (* (numerator x) (/ (denominator x))) to
(* (/ (denominator x)) (numerator x)), in order to respect the
fact that unary-/ is invisible with respect to binary-*.
See loop-stopper.

The `preprocess' process in the waterfall (see hints for a
discussion of the :do-not hint) has been changed so that it works to
avoid case-splitting. The `simplify' process refuses to force
(see force) when there are if terms, including and and or
terms, in the goal being simplified.

The function apply is no longer introduced automatically by
translation of user input to internal form when functions are called
on inappropriate explicit values, e.g., (car 3).

The choice of which variable to use as the measured variable in a
recursive definition has been very slightly changed.